Secure Communication in a Multi-antenna Wiretap Channel with a Reconfigurable Intelligent Surface

Reconfigurable Intelligent Surfaces (RIS) are being considered as a candidate technology for next generation of wireless systems (6G), for improving reliability and range extension owing to their low cost and complexity, and suitability for high-frequency schemes. RIS terminals can effectively operate as relaying nodes for transmitted signals, while saving the energy needed for Radio Frequency (RF) transmission. This is implemented by deploying controllable reflecting arrays of meta-surface elements, whose phase can be adjusted by applying fast switching voltage. RIS thus creates a valuable resource in wireless communication: the control over the channel realizations from the RIS relays to the receiver. In this work, we investigate the impact of this resource on improving secure communication rates between a transmitter (base station) and a receiver (end user), in the presence of an external eavesdropper. We consider a multi-antenna wiretap channel aided with a RIS array, and derive the achievable secure degrees of freedom for various number of antennas at different terminals. Our achievability scheme relies on designing the RIS beamforming such that the signals received at the eavesdropper experience destructive interference. Compared to our previous work on multi-antenna wiretap channel with a multi-antenna helper, where active user cooperation enables/improves secure communication rates, here secure communication is enabled/improved by utilizing passive terminals (deployed at low cost).